WL#5228: Update the Plugin Documentation

Update the documentation on the Plugin functions and architecture.
There are some missing elements to the current Extending MySQL content,
particularly on the plugin interface.
This item will address some of the content previously covered by these bugs:
http://bugs.mysql.com/bug.php?id=46851
http://bugs.mysql.com/bug.php?id=47009
http://bugs.mysql.com/bug.php?id=47169
http://bugs.mysql.com/bug.php?id=34413